Cellular ceramic plates with asymmetrical cell structure and manufacturing method thereof

ABSTRACT

A method for the continuous production of a cellular ceramic plate having asymmetric cells comprising thermally treating ceramic particles and a blowing agent in a foaming furnace while conveying said ceramic particles and said blowing agent at a first speed thereby forming a cellular ceramic plate, and annealing said cellular ceramic plate in an annealing lehr by cooling it down while conveying it at a second speed, larger than said first speed, thereby stretching and cooling said cellular ceramic plate.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for thecontinuous production of cellular ceramic products (e.g. cellularceramic plates such as glass foam plates). The present invention alsorelates to cellular ceramic products obtained by such method.

TECHNICAL BACKGROUND

There are several methods known for the manufacture of cellular ceramicmaterials involving foaming. Examples are:

-   a) insertion (incorporation e.g. injection) and mechanical    distribution of gases in a low viscosity melt.-   b) Release and expansion of dissolved gases in a low viscosity melt    under vacuum.-   c) Insertion (incorporation) of foaming agents in a melt.-   d) Mixing glass powder with a foam agent and subsequent heating.

In the case of the last glass foaming process, the manufacturingequipment typically comprises a foaming furnace with a belt carrying theglass powder and foam, and a powder loading apparatus. The foaminginvolves the foaming of either thick or thin plates.

SUMMARY OF THE INVENTION

The present invention results from the observation that stretching ofcellular ceramic products (such as foam glass) during production bringsabout changes in the physical properties of the final material such asthe thermal insulation of the cellular ceramic product (such as glassfoam). Adaptation of these physical properties is therefore possible.This is advantageous as it permits the properties of the glass to betailored to customer needs (e.g. stretching lowers the thermalconductivity, i.e. improves insulation properties)

In a first aspect, the present invention relates to a method for thecontinuous production of a cellular ceramic plate (e.g. a one-piececeramic plate) comprising:

-   -   a) thermally treating ceramic particles and a blowing agent in a        foaming furnace while conveying said ceramic particles and said        blowing agent at a first speed to thereby form a cellular        ceramic plate, and    -   b) annealing said cellular ceramic plate in an annealing lehr by        cooling it down while conveying it at a second speed, larger        than said first speed, thereby stretching and cooling said        cellular ceramic plate.

In an embodiment of the first aspect, the present invention relates to amethod wherein prior to step (b), the (one-pieced) cellular ceramicplate is transferred from said foaming furnace to said annealing lehrvia an intermediate conveyor at a third speed higher or equal to saidsecond speed. This is advantageous as it enables the stretching of thecellular ceramic plate to be performed in a zone of relatively hightemperature without the need for the annealing conveyor (secondconveyor) to be resistant to said relatively high temperature.Stretching at relatively low temperatures induces more stress in thefoam than stretching at relatively high temperatures. Preferably, onlythe intermediate conveyor, which can be shorter (e.g. much shorter) thanthe annealing conveyor, is adapted to be resistant to said relativelyhigh temperature. Since the annealing requires a relatively longconveyor, it is economical to use an annealing conveyor which is notadapted to be resistant to relatively high temperatures (e.g. when anintermediate conveyor is used, a second conveyor resistant to atemperature up to 600° C. is enough and there is no need to use a secondconveyor resistant to a temperature up to 800° C. or 900° C. in the longannealing lehr).

In an embodiment of the first aspect, the present invention relates to amethod wherein the difference between the second speed and the firstspeed may be 25% or less of the first speed, preferably between 1 and25%, more preferably between 2 and 20%, most preferably between 3 and15%. Stretching in these ranges provides improvement in heat insulation(lower k-values) while simultaneously keeping the amount of breakagerelatively low. In general, to decrease breakage at high speeddifferences, a higher stretching temperature is helpful.

In an embodiment of the first aspect, the present invention relates to amethod wherein the difference between the third and second speed isbetween 0 and 10% (or between 1 and 10%), 0 to 5% (or 1 to 5%) beingpreferred. Some pre-stretching is advantageous as it allows theone-pieced cellular ceramic plate to shrink during the annealing,thereby releasing stress and reducing the fracture tendency.

In an embodiment of the first aspect, the present invention relates to amethod wherein the stretching is between 3 and 15%. This is advantageousas it is in this range that the ceramic cellular plate formed hassimultaneously acceptable compressive strength and improved insulationproperties in comparison with an otherwise identical non-stretchedcellular ceramic plate.

In an embodiment of the first aspect, the present invention relates to amethod wherein the cellular ceramic plate may be a glass foam plate.

The foaming step may produce open or closed cells. For insulatingpurposes closed cells are preferred. In the case of foamed glass, opencells can be obtained by addition of some crystalline material (such ase.g. TiO₂) to the amorphous glass powder. For instance, adding around 1%TiO₂ during grinding (e.g. in a ball mill) of the glass can lead to 100%open cells in a glass foam. When closed cells are required, the additionof TiO₂ or similar crystalline material is preferably avoided.

In a second aspect, the present invention relates to an apparatus forthe continuous production of a cellular ceramic plate comprising:

-   -   a) a foaming furnace for thermally treating ceramic particles        and a blowing agent while conveying at a first speed to thereby        form a cellular ceramic plate, and    -   b) an annealing lehr for annealing said cellular ceramic plate        by cooling it down while conveying it at a second speed, larger        than said first speed, thereby stretching and cooling said        cellular ceramic plate.

In other words, the second aspect of the present invention relates to anapparatus for the continuous production of a cellular ceramic platecomprising:

-   -   a) a foaming furnace for thermally treating ceramic particles        and a blowing agent, said foaming furnace comprising a first        conveyor adapted for conveying at a first speed (i.e. linear        speed) while heating said ceramic particles and said blowing        agent to form a cellular ceramic plate, and    -   b) an annealing lehr for annealing said cellular ceramic plate        by cooling it down, said annealing lehr being downstream from        said foaming furnace and comprising a second conveyor adapted        for conveying said cellular ceramic plate at a second speed        (i.e. linear speed), larger than said first speed.

For the purpose of obtaining a second linear speed higher than the firstlinear speed, independent driving means may be provided for said firstand said second conveyor.

In an embodiment of the second aspect, the apparatus further comprisesan intermediate conveyor prior to the second conveyor for transferringthe cellular ceramic plate from said first conveyor (e.g. from saidfoaming furnace) to said second conveyor (e.g. from said annealinglehr). The presence of the intermediate conveyor between said first andsecond conveyors permits the use of a second conveyor with lower heatresistance than were said second conveyor to be directly adjacent to thefirst conveyor. This is particularly advantageous in view of theresulting reduced costs per unit length, which is important in view ofthe relatively large length of the second conveyor when compared to theintermediate conveyor.

In embodiments of the second aspect, where an intermediate conveyor ispresent. It may be adapted for conveying at a third linear speed higheror equal to said second speed. In other words, it may be driven at saidthird speed higher or equal to said second speed. This is advantageousas it permits the stretching of the cellular ceramic plate to beperformed in a zone of relatively high temperature without the need forthe annealing conveyor (second conveyor) to be resistant to relativelyhigh temperatures. A stretching performed at a relatively hightemperature leads to less stress than a stretching performed at arelatively low temperature.

For the purpose of obtaining a third linear speed higher than saidsecond speed, independent driving means may be provided for said thirdconveyor.

In embodiments of the second aspect of the present invention, the firstand second conveyors may be adapted for being driven in such a way thatthe difference between the second speed and the first speed is 25% orless of the first speed, preferably between 1 and 25%, more preferablybetween 2 and 20%, most preferably between 3 and 15%. In other words,the first and second conveyors may be driven in such a way that thedifference between the second speed and the first speed is 25% or less,preferably between 1 and 25%. In some embodiments this difference can bebetween 5 and 25%.

In embodiments of the second aspect where an intermediate conveyor ispresent, the difference between the third and second speed may bebetween 0 and 10%, with between 0 and 5% being preferred.

In embodiments of the second aspect of the present invention, the firstconveyor may be adapted to be resistant to higher temperature than thesecond conveyor. This is advantageous as the temperature in the foamingzone is higher than the temperature in the annealing zone.

In embodiments of the second aspect of the present invention, the firstconveyor may be adapted to be resistant to a temperature up to 800° C.,preferably up to 900° C. and the second conveyor may be adapted to beresistant to a temperature up to 600° C. These temperatures are typicalmaximal temperatures for the foaming and the annealing steprespectively.

In embodiments of the second aspect of the present invention, theintermediate conveyor may comprise rolls. The preferred distance betweentwo rolls can vary in function of many parameters and is preferably setvia trial and error. Typically, this distance can be from 0.2 m to 1.5m. In some embodiment, the distance between two rolls can be from 0.2 to0.4 m. In other embodiments, the distance between two rolls can be atleast 0.8 m and less than 1.5 m.

This is advantageous because within this range, the distance is largeenough to reduce the number of rolls and therefore the number offriction zones between the cellular ceramic plate and the conveyors.This friction causes dust. For larger distances between the rolls, therisk of jam-up in case of breakage of the cellular ceramic plate becomessignificantly higher. A roll conveyor is advantageous because it iseasier to construct than belt conveyors and it leads to less jam up ofbroken cellular ceramic plates when used at relatively high temperature(for foam glass, this is especially true above 450° C.).

In embodiments of the second aspect of the present invention, theintermediate conveyor may be situated at the beginning of the annealinglehr or in an intermediate lehr situated between the foaming furnace andthe annealing lehr.

It is advantageous to adapt the apparatus in such a way thatsubstantially no temperature gradient (e.g. at least perpendicularly tothe conveying direction) exists in the zone of the apparatus wherestretching occurs (e.g. between the first and the second conveyor if nointermediate (third) conveyor is present or between the first and theintermediate (third) conveyor if an intermediate conveyor is present).This way, fracture of the cellular ceramic plate during and afterproduction is minimised.

In embodiments of the second aspect of the present invention where anintermediate conveyor is present, the intermediate (third) conveyor maybe resistant to temperatures in the range 600° C.-800° C., preferably inthe range 600° C.-900° C.

This is advantageous as the presence of an intermediate (third) conveyorensures a more reliable transition between the first conveyor and thesecond conveyor.

In a third aspect, the present invention relates to cellular ceramicplate having a cell structure, whereby the cells are asymmetrical, e.g.elongated. In an embodiment, the longest dimension of the cells (e.g.the length) may on average be larger than the shortest dimension of thecell (e.g. its dimension perpendicular to the surface of the plate(height)). In embodiments of the present invention this ratio betweenthe average largest dimension of the cells and the average shortestdimension of the cells may be from 1.2 to 2.5. An advantageous ratio hasbeen found to be between 1.2 and 1.6 as it provides a trade off betweeninsulation properties and mechanical properties. For the purpose of thepresent invention, this difference in length between the average largestdimension and the average smallest dimension has been measured withultrasonic measurements. For instance, the ratio of ultrasonic transittimes measured lengthwise (in the direction of the conveying andstretching) on ultrasonic transit times measured heightwise(perpendicular to the glass foam surface) has been found to be about 1.4for glass foam exhibiting a thermal conductivity of 0.042 W/mk at 10° C.with a density of 115 kg/m³.

In an embodiment of the third aspect, the cellular ceramic plate mayhave an asymmetrical cell structure and may be obtainable by any of themethods of the first aspect of the present invention.

In an embodiment of the third aspect, the cellular ceramic plate may bea glass foam plate and/or be a closed cell foam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an apparatus according to an embodiment ofthe present invention.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

Although the present invention will be described in connection withcertain embodiments, it is not intended to be limited to the specificform set forth herein. Rather, the scope of the present invention islimited only by the accompanying claims. In the claims, the term“comprising” does not exclude the presence of other elements or steps.Additionally, although individual features may be included in differentclaims, these may possibly be advantageously combined, and the inclusionin different claims does not imply that a combination of features is notfeasible and/or advantageous. In addition, singular references do notexclude a plurality. Thus, references to “a”, “an”, “first”, “second”etc. do not preclude a plurality. Furthermore, reference signs in theclaims shall not be construed as limiting the scope.

Definitions:

Types of “Cellular Ceramics” include but are not limited to carbonfoams, glass foams, and cellular concrete. Glass foams have acombination of unique qualities including rigidity, compressivestrength, thermal insulation, non-flammability, chemical inertness,water/steam resistance, insect/rodent resistance, and are generallylightweight. Glass foams are generally formed by the action of agas-generating agent (e.g. foaming agent), which is mixed with groundglass (i.e.; glass particles). This mixture is heated to a temperatureat which the evolution of gas from the foaming agent occurs within thesoftened glass. The gas evolved creates bubbles which form the cells(e.g. pores) in the final glass foam. Cellular ceramics according to thepresent invention preferably have a density which is from 2 to 45%,preferably 3 to 25%, more preferably 4 to 10%, of the density of thecorresponding plain (non-cellular) ceramic. In the case of foam glass,the density of the glass obtained is preferably from 50 to 1000 Kg/m³,preferably from 75 to 600 Kg/m³, most preferably from 90 to 250 Kg/m³ or100 to 250 Kg/m3.

As used herein and unless provided otherwise, the term “plate”, whenrelating for instance to cellular ceramic plates, relates to athree-dimensional object, wider than thick and of any length. In thecontext of the first aspect of the present invention, the term “plate”refers to a continuous one pieced plate until it is cut after or at theend of the annealing step. The term “plate” as used in the context ofthe third aspect of the present invention, refers either to saidcontinuous one pieced plate or to a shorter plate as obtained aftertransversal cutting of said continuous plate.

As used herein and unless provided otherwise, the term “resisting” whenrelating to a temperature applied to a conveyor, means that the conveyordoes not substantially deform when exposed to said temperature for anextended period. For instance, when the conveyor is a belt conveyor, theelongation of the belt should preferably not be more than I% over a 120days period at said temperature.

The term foaming furnace, as used in disclosing the present invention,means a furnace in which cellular ceramic (e.g. foamed glass) isproduced.

In a first aspect, the present invention relates to a method for thecontinuous production of a (one-piece) cellular ceramic plate. Bycontinuous production, is meant the production of a one-piece(continuous) cellular ceramic plate, as opposed to batch-wise productionmethods such as e.g. moulding. In the continuous production processesaccording to the first aspect of the present invention, a single pieceof cellular ceramic plate is produced which is cut into plates ofwell-defined length at the end of the annealing step or after theannealing step.

In embodiments of the present invention, said method comprises the stepsof: (a) thermally treating ceramic particles and a blowing agent in afoaming furnace thereby forming a cellular ceramic plate, and (b)annealing said cellular ceramic plate in an annealing lehr by cooling itdown. In a preferred embodiment, the cellular ceramic plate is a glassfoam plate.

In an embodiment, the thermal treatment can be heating up the ceramicparticles and the blowing agent to a temperature high enough to inducethe formation of a cellular ceramic material. This temperature can, forinstance, be comprised between 650 and 850° C. in the case of glassfoam, with from 700 to 800° C. being preferred. The ceramic particlesmay have any shape, size or aspect ratio known in the art to permit theproduction of cellular ceramic bodies. The ceramic particles specificsurface area is preferably from 0.5 m²/g to 1 m²/g as measured byBrunauer Emmett Teller (BET)-analysis. Blowing agents usable inembodiments of the first aspect of the present invention comprise anyblowing agent known in the art enabling the production of cellularceramic bodies. They can comprise but are not limited to carbon blackand carbonates (e.g. calcium carbonate or sodium carbonate). Theproportion of blowing agent to ceramic particles can be any proportionknown in the art to permit the production of cellular ceramic bodies.Preferably, it is between 0.1% and 2%. For carbon black, it ispreferably from 0.2 to 0.6% and particularly preferably from 0.3 to0.5%. For carbonates, it is preferably between 0.7 and 1.3%, preferablyfrom 0.8 to 1.2%.

In embodiments of the present invention, the annealing step may be aslow temperature decrease according to a prescribed temperature profile.

Although this is not the object of the present invention, thetemperature profile of the whole foaming and annealing process has aninfluence on the appearance of defects in the foamed glass productobtained via a stretching procedure as described herein. The fine-tuningof this temperature profile is a question of trial an error and is wellwithin the skills of the person skilled in the art without undueexperimental effort. In general, the purpose of these temperatureprofiles is to relax at a smaller temperature gradient to reduceresidual strain. It is also advantageous to have smooth temperaturetransitions between the different sections of the apparatus.

In a preferred embodiment of the first aspect of the present invention,step (a) is performed while conveying said ceramic particles and saidblowing agent at a first speed thereby forming a ceramic plate and step(b) is performed while conveying said cellular plate obtained in (a) ata second speed, larger than said first speed, thereby stretching saidcellular plate. The first speed can, for instance, range from 1 to 100cm/min depending upon the ceramic material foamed and the platethickness to be produced. For instance, it can range from 1 to 15 cm/minin some embodiments. In embodiments of the present invention, the use ofa first conveyor conveying the foam at a first speed in the foaming zoneand the use of a second conveyor in the annealing zone conveying thefoam at a second speed higher than said first speed, permits thecellular ceramic foam, e.g. the glass foam, to be stretched.

In an embodiment of the present invention, the speed of the secondconveyor, i.e. the speed of the annealing conveyor, is higher than thespeed of the foaming conveyor (i.e. the first conveyor). Especiallyduring the production start (e.g. when the production has beeninterrupted and must be restarted), it is very advantageous to have adistinctly higher (e.g. between 3 and 20% higher, preferably between 4and 20% higher, more preferably 7%-20% higher, for instance 8% orhigher) second conveyor speed in the lehr than in the first conveyor inthe foaming zone. After the production start, the second conveyor speedcan be reduced to be, for instance, around 3%. The difference betweenthe first speed and the second speed is preferably 25% or less, morepreferably between from 3 to 25% or from 5 to 25%. As an example, it hasbeen shown that stretching a continuous glass foam string duringproduction by up to 20%, e.g. up to 10%, reduces the k-value anddecreases the compressive strength in embodiments of the presentinvention, the preferred stretching is between 3 and 25% (e.g. between 5and 25%).

In an embodiment of the present invention, the stretching of the foam isobtained by using separate conveyors for the foaming and for theannealing of the foamed glass.

In an embodiment of the first aspect of the present invention, prior tostep (b) and after step (a), the cellular plate is transferred from saidfoaming furnace to said annealing lehr via an intermediate conveyor.Preferably, said intermediate conveyor conveys at a third speed higheror equal to said second speed. Preferably, the difference between thethird speed and the second speed is between 0 and 10% of the secondspeed. In another embodiment of the present invention, the intermediateconveyor (e.g. the rolls of the intermediate conveyor when it comprisesrolls) may be coupled with the second conveyor in such a way that thelinear speed of the intermediate conveyor equals that of the secondconveyor. As a consequence, the stretching occurs between the firstconveyor (e.g. a foaming belt) and the intermediate conveyor (e.g. the1^(st) roll of the intermediate conveyor), where the temperature ishigher. As an optional feature, a pre-stretch of the foam by using aspeed for the intermediate conveyor up to a value a few % of the secondspeed higher (e.g. between 1 and 10%, for instance 5%) than the value inthe annealing lehr is performed between the first and the intermediateconveyor. This allows the foam to shrink in the lehr and the finalstretching is the stretching due to the difference in speed between thefirst conveyor and the second conveyor. Shrinking of the ceramiccellular material in the lehr reduces stress and breakage.

For all embodiments of the first aspect of the present invention, it ispreferred that the temperature distribution across the width of thecellular ceramic plate is as uniform as possible in the zone where thestretching occurs. In embodiments, the temperature distribution acrossthe width of the cellular ceramic plate span 20° C. or less in the zonewhere the stretching occur. This can be obtained, for instance, byisolating said zone from the rest of the apparatus (avoiding drafts(e.g. air and flue gas currents)) and/or by adapting the position ofheaters with individual temperature control. In an embodiment, the zonewhere the stretching occurs (e.g. the zone in between the first conveyorand the intermediate conveyor) is adapted to experience a local minimumof currents. This means that zones situated directly upstream ordownstream from said zone where the stretching occurs, experience moredrafts than said zone where the stretching occurs.

In a second aspect, the present invention relates to an apparatus forthe continuous production of a cellular ceramic plate. This apparatus isadapted to perform the steps of the method of the first aspect. Theapparatus of the present invention comprises a foaming furnace and anannealing lehr. The foaming furnace is suitable for thermally treatingceramic particles and a blowing agent. The treatment temperature mayvary depending upon the nature of the particles used. For instance, inthe case of glass particles it can be between 600° C. and 950° C. and ispreferably between 650° C. and 800° C. during most of the foamingprocess. The annealing lehr is suitable for annealing the cellularceramic plate by cooling it down in a controlled manner. The annealinglehr is downstream from the foaming furnace. The apparatus alsocomprises at least two conveyors; a first conveyor and a secondconveyor. The conveyor used for the foaming will be hereafter referredto as the first conveyor. The first conveyor is situated in the foamingzone (e.g. it is comprised in said foaming furnace). A suitable conveyorfor this purpose is an endless metallic belt with holes filled with asuitable ceramic material. The conveyor used for the annealing will behereafter referred to as the second conveyor. The second conveyor iscomprised in the annealing lehr. A suitable conveyor for the annealinglehr can for instance be a belt or rolls.

The length of the foaming conveyor can be for instance (in the case ofglass foaming) from 35 to 75 m, e.g. from 45 to 55 m. The length of theannealing conveyor can be for instance (in the case of glass foaming)from 150 to 300 m, preferably from 200 to 280 m. In general, thesedimensions can be made smaller or larger by decreasing or increasing theconveying speed respectively. Much smaller (see the example below on apilot size line) or larger dimension are therefore useable. Inembodiments of the present invention, the ratio between the length ofthe second conveyor and the length of the first conveyor is from 2 to 8.

In a preferred embodiment of the second aspect of the present invention,the first conveyor is adapted to convey at a first speed while thesecond conveyor is adapted to convey at a second speed, higher than saidfirst speed. Preferably, the first conveyor and the second conveyor areadapted for being driven in such a way that the difference between thesecond speed and the first speed is 25% or less of the first speed, morepreferably from 1 to 25% and most preferably from 3 to 5%. Inembodiments, this difference can be from 5 to 25%. In embodiments of thepresent invention, stretching implies a higher speed for the secondconveyor than for the first conveyor and therefore a longer lehr thanwere there to be no stretching. If 20% of stretching is required, a 20%longer second conveyor is preferably used. In other words, the lenath ofthe conveyor is preferably proportional to the required stretching.

In an embodiment of the second aspect of the present invention, thefirst conveyor is preferably adapted to be resistant to highertemperatures than said second conveyor. More preferably, the firstconveyor is adapted to be resistant to a temperature up to 800° C. oreven 900° C. or 950° C. A suitable first conveyor can for instance be ametallic mesh belt filled in with a suitable ceramic (e.g. a ceramicresistant to said temperature without shrinking substantially). Morepreferably, the second conveyor is resistant to higher temperatures upto e.g. 800° C., preferably 900° C. if no intermediate conveyor is usedbetween the first conveyor and the second conveyor and to a lowertemperature (e.g. up to 600° C.) if an intermediate conveyor is usedbetween the first conveyor and the second conveyor. In an embodimentwherein an intermediate conveyor is used, the second conveyor may beadapted to be resistant to a temperature up to 600° C.

The second conveyor being relatively long, and the temperature at theend of the foaming zone (i.e. foaming furnace) being relatively high (upto 800° C. or even up to 900° C. or 950° C. in the case of glass foamplates), it is advantageous to have an intermediate conveyor between thefoaming zone and the annealing conveyor, which is resistant torelatively high temperature (e.g. in the range 600° C.-800° C.). Hence,in some embodiments of the present invention, it is advantageous to useone or more intermediate conveyors between the first conveyor and thesecond conveyor. In embodiments of the second aspect of the presentinvention, the apparatus further comprises at least a third conveyor(also referred to as intermediate conveyor(s) in the rest of the text).Preferably a single intermediate conveyor is used and although the restof the description will refer to a single intermediate conveyor, itapplies mutatis mutandis to multiple intermediate conveyors. Thepresence of this intermediate conveyor permits the use of a lesstemperature-resistant and therefore cheaper second conveyor (e.g. oneonly resistant to temperatures up to 600° C. in the case of glass foamplates production). This is particularly advantageous in view of therelatively long length arid therefore high cost of the second conveyor.Said intermediate conveyor is preferably adapted to convey at a thirdspeed equal or higher than the second speed. In an embodiment of thepresent invention, a separate driving system is provided on theintermediate conveyor (e.g. on the rolls of the intermediate conveyor).As a consequence, the equipment is able to generate a different linearspeed for the intermediate conveyor than for the first or secondconveyor. More preferably, the difference between the third and secondspeed is between 0 and 10%, preferably between 0 and 5%. In a preferredembodiment of the present invention, when an intermediate conveyorcomprising rolls is used between the first and the second conveyor, therolls may be driven at such a speed that the conveying speed of theintermediate conveyor is the same or up to 10%, preferably 5% fasterthan the conveying speed of the second conveyor. Preferably, theintermediate conveyor is resistant to temperatures in the range 600°C.-800° C., i.e. up to 800° C., more preferably up to 850° C. The lengthof the intermediate conveyor can for instance be from 2% to 30% of thelength of the second conveyor, preferably being from 3% to 20% of thelength of the second conveyor. The intermediate conveyor preferablycomprises rolls. This is advantageous as it is easier and cheaper tobuild and breakage is less likely with rolls when the ceramic (e.g.glass) is at a temperature high enough for it to be viscoelastic. Thepreferred distance between two rolls is better determined by trial anderror as it depends upon many parameters. Typically it can range from0.2 to 1.2 m. A preferred distance is from 0.2 to 0.4 m, with in someembodiments 0.6 m or more and less than 1.5 m being used, in otherembodiments, 0.8 m or more and less than 1.2 m can be used. In yet otherembodiments, between 0.9 and 1.2 m can be used. In some embodiments, auseful value has been found to be 0.3 m. In other embodiments, a usefulvalue has been found to be 1 m. The intermediate (third) conveyor isplaced prior to (i.e. upstream from) the second conveyor. It ispreferably situated at the beginning of the annealing lehr or in anintermediate lehr situated between the foaming zone furnace and theannealing lehr. Said intermediate conveyor is suitable for transferringa cellular ceramic plate from the foaming furnace to the annealing lehr.In an embodiment of the present invention, transversal temperaturegradients (temperature differences across the plate) where stretchingoccurs are preferably avoided. Preferably, the transversal temperaturegradient (temperature difference across the plate) where stretchingoccurs is of 20° C. or less. This can for instance be achieved byinstalling heaters with separate temperature control at the appropriateplaces.

In a third aspect, the present invention relates to cellular ceramicplates having a cell structure, whereby the cells are asymmetrical. Theplate obtained in the method of the first aspect is a one-piececontinuous plate that can be cut in any desired dimensions after or atthe end of the annealing step. Due to stretching, properties in thefinal material are different from those of non-stretched plates. Thedimension and shape of the cells within a cellular ceramic plate ((e.g.a glass foam plate) stretched according to an embodiment of the presentinvention are as follows: the average diameter of the cells ispreferably smaller than 1 mm and the cell shape will on average beasymmetrical with one dimension larger than the other. Preferably, onedimension is larger than the other by a factor in ultrasonic transittime comprised between 1.2 and 1.6, preferably 1.3 and 1.5, e.g. about1.4.

In an embodiment of the third aspect, the present invention relates tocellular ceramic plates obtainable by any of the methods of the firstaspect of the present invention.

EXAMPLES Example 1 Pilot Line

A Glass foam plate was produced according to the first aspect of thepresent invention. For this example, various glass foam plates wereproduced with an apparatus comprising a powder loading apparatus, afoaming furnace including a first conveyor, an intermediate zoneincluding an intermediate (third) conveyor and an annealing lehrincluding a second conveyor. The glass powder was applied on the foamingconveyor in an amount of 8000 cm²/g. The foaming oven was 10 m long. Thefirst conveyor was a powder-tight refractory steel belt filled with aheat-stable ceramic material. Its linear speed was ca. 3 cm/min. Thetemperature in the foaming furnace was between 650 and 670° C. at thebeginning of the furnace and between 750 and 770° C. at the end of thefurnace. The intermediate (third) conveyor was a set of water cooledrolls. The temperature in the intermediate zone was between 650 and 680°C. at the beginning of the intermediate zone and reached a maximum of800° C. between the beginning and the end of the zone and was around700° C. at the end of the intermediate zone. The length of theintermediate conveyor was 1 meter. Its rolls were driven at a speedabout 5% higher than the speed of the second conveyor. The secondconveyor was another set of rolls (the use of a belt would also havebeen suitable) and the temperature in the annealing lehr was about 600°C. at the beginning of the lehr down to room temperature (20-40° C.) atthe end of the lehr. Its length was about 22 m. The second conveyor hada linear speed 5, 10 and % above the speed of the first conveyor leadingto foam-glass plates having a density of 105 Kg/m³ with stretching of 5,10 and 15% respectively. The foam-glass plates could thereafter be sawedlaterally and/or horizontally and/or transversally.

The relative speeds of the first, second and intermediate conveyors inthese examples were as follow:

The first speed was always about 3 cm/min. For a stretch of 5, 10 or15%, the second speeds were respectively 5, 10 or 15% higher than thefirst speed. The third speed (i.e. the speed of the intermediateconveyor) was 5% higher than the second speed. For a stretching up to15%, we obtained the following results in table 1 below.

TABLE 1 Stretch [%] compressive strength [N/mm²] k-value [W/mK] 5 0.90.0415 10 0.77 0.0413 15 0.7 0.0408

The results in table 1 show that stretching brings about a decrease inthe k-value and a decrease in: the compressive strength. Other densitiesor ceramics would give different results.

We obtained improvement in the mechanical properties with a conveyingspeed for the second conveyor 20% higher than for the conveying speed inthe foaming furnace. This led to a stretch of 20%. In this way, weobtained foams with an approximate thickness of 16 cm at 120 kg/m³.

With a set-up wherein an intermediate conveyor is used at a speed higherthan the speed of the second conveyor and therefore a pre-stretching upto a first value, e. g. 25% with a net stretching equal to a secondvalue lower than the first value, e.g. 20%, it was possible to anneal a16 cm thick foamed glass sheet with a 120 kg/m³ density without lehrbreakage, only 10% delayed breakage and a defect free bottom at 3.18cm/min for the first conveyor.

FIG. 1 schematically shows an apparatus according to an embodiment ofthe present invention.

In this FIGURE, a first conveyor 1, a second conveyor 2, and anintermediate conveyor 5 are shown. The first conveyor 1 conveyed thefoaming glass through the foaming furnace 3 and transferred thefoamed-glass ribbon to the intermediate conveyor 5. The intermediateconveyor 5 conveyed the foamed glass ribbon through the intermediatelehr 6 and transferred the foamed-glass ribbon to the second conveyor 2.The second conveyor 2 conveys the foamed glass ribbon through theannealing lehr 4.

1-17. (canceled)
 18. An apparatus for the continuous production of aone-piece continuous cellular ceramic plate comprising: a) a foamingfurnace arranged to treat in one continuous piece ceramic particles anda blowing agent, said foaming furnace comprising a first conveyoradapted to convey at a first speed while heating said one piececontinuous ceramic particles and said blowing agent to form a one-piececontinuous cellular ceramic plate, wherein the first conveyor comprisesan endless belt; b) an annealing lehr arranged to anneal said one-piececontinuous cellular ceramic plate by cooling said one-piece continuouscellular ceramic plate down, said annealing lehr being locateddownstream from said foaming furnace and comprising a second conveyorconfigured to convey said one-piece continuous cellular ceramic plate ata second speed, larger than said first speed; and c) an intermediateconveyor arranged between the first conveyor and the second conveyor totransfer the one-piece continuous cellular ceramic plate from saidfoaming furnace to said annealing lehr, wherein the intermediateconveyor is configured to convey at a third speed larger than saidsecond speed, wherein the intermediate conveyor comprises rolls, andwherein the intermediate conveyor is located at the beginning of theannealing lehr or in an intermediate lehr located between the foamingfurnace and the annealing lehr.
 19. The apparatus according to claim 18,wherein said first and second conveyors are driven in such a way thatthe difference between the second speed and the first speed is between0.1% and 25% of the first speed.
 20. The apparatus according to claim19, wherein the difference between the third and second speed is between0.1% and 10% of the second speed.
 21. The apparatus according to claim18, wherein the first conveyor is resistant to higher temperatures thansaid second conveyor.
 22. The apparatus according to claim 21, whereinthe first conveyor is resistant to a temperature up to 900° C. andwherein said second conveyor is resistant to a temperature up to 600° C.23. The apparatus according to claim 18, wherein the intermediateconveyor is resistant to temperatures in the range 600° C.-800° C. 24.The apparatus according to claim 18, wherein the first conveyor isresistant to higher temperatures than said intermediate conveyor andsaid intermediate conveyor is resistant to higher temperatures than saidsecond conveyor.
 25. The apparatus according to claim 24, wherein thefirst conveyor is resistant to a temperature up to 900° C. and whereinsaid second conveyor is resistant to a temperature up to 600° C. andwherein said intermediate conveyor is resistant to temperatures in therange 600° C.-800° C.